This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Improper maintenance of ploidy (chromosome number) is a hallmark of many cancer cell types, and is thought to contribute to malignancy in cancer cell populations. In a number of tumors tetraploidy, exactly twice the normal complement of DNA, precedes a more degenerate aneusomic state in which individual chromosomes appear to be lost or gained at random. This condition is referred to as the chromosome instability, or "CIN" phenotype. How the initial tetraploid state develops is not known. In a prior funding period, we began an investigation of the behavior of chromosomes in tetraploid cells. Here we propose studies in which we look specifically at the development of tetraploidy. Several lines of evidence strongly suggest a direct connection between defective sister chromatid cohesion (SCC) and aneuploidy in vertebrates. Here we propose that chromosome instability develops in two steps in response to loss of cohesion. In the first step, reduced cohesion results in the formation of tetraploid cells through mitotic slippage. In the second step, tetraploid cells provide a permissive environment for the missegregation of whole chromosomes, leading eventually to chromosome instability. In the following proposal we describe a set of experiments with which we will test the first part of this model, that failures in sister chromatid cohesion lead to tetraploidy through mitotic slippage. We will do this in two ways. First, we will generate cells or cell lines defective in cohesion and monitor the rate of tetraploid formation in these cells. Secondly, we will determine whether a weakened or inactivated spindle checkpoint accelerates the development of tetraploidy in cells with compromised cohesion. The data from the experiments described in this proposal will provide important insight into how failures in cohesion might lead to chromosome instability, and thus contribute to tumor development and malignancy.